The present invention relates to curved multi-planar reformatting of three-dimensional volume data sets, particularly volume data sets representing patient images, and user interfaces for computer-implemented curved multi-planar reformatting systems.
Modern patient imaging methods, such as computed tomography (CT) and magnetic resonance imaging (MRI), generate large three-dimensional volume data sets representing the patient's body. These volume data sets are highly detailed and allow complex studies of the body to be made. However, a typical volume data set will contain details of many body parts which are not relevant for a particular study, or which obscure important features. Data processing methods are hence used to manipulate the data set and present a viewer with an image which shows the items of interest in a useful and meaningful manner.
The images are of necessity two-dimensional renderings of parts of the three-dimensional data set. They may take the form of planar slices through the volume. However, the anatomical feature of interest is almost always non-planar, so cannot be shown fully in a single slice. For example, a surgeon may wish to study an artery along its entire length. The artery will follow a complex path in three-dimensions, so that any slice through the volume around the artery will only show part of it. The same problem arises for other curved anatomical structures, such as bronchi, the colon, the spine, and dental structures. Various data processing techniques have been developed to address this issue. Vilanova et al [1] describe a virtual endoscopy method to visualise tubular curved structures, which uses algorithms to calculate the path of a central axis of the structure from the data set. This axis is then used as a camera path for a virtual endoscopic camera.
An alternative approach for viewing curved structures is a computer-implemented technique known as curved Multi-Planar Reformatting (MPR), or Curved Planar Reformatting (CPR). This is a computer-implemented method in which a curve of interest is defined through the volume data set, for example along the central axis of an artery. Data processing is then used to calculate the co-ordinates of a curved sheet which follows the bends of the curve. The curve is thus wholly contained within the plane of this sheet. The sheet is flattened out and presented to the user as a two-dimensional image showing the entire path of the curve, in this case, a longitudinal cross-section through the artery.
Curved MPR is a complex spatial technique. To obtain useful images, it is necessary to define coordinate systems or specify directions within the volume data set with respect to which the curve and the curved sheet can be oriented. One way of achieving this is to obtain some criteria from the user, and allow the software to make assumptions regarding other criteria. This makes for simple software, and increased ease of use for the user, because user input is minimised. However, in some circumstances, the assumptions made may be inaccurate or wholly incorrect. This can occur for example if ambiguities are present in the data set so that two or more assumptions appear equally valid. If the user recognises from the final curved MPR image that this has happened, he will have to repeat the imaging process, perhaps using different input criteria to avoid the ambiguity. This is clearly undesirable. Worse, though, is that there may be instances when the user fails to identify any problem. This can lead to erroneous clinical decisions being made, which puts patients at risk.
One prior art system [2, 3, 4] initially presents the user with three orthogonal views through the volume data set, each bearing a cross-hair. Selection by the user of one of the cross-hairs defines a general direction of the curve through the volume, while the underlying view defines a direction from which the user desires to view the resulting curved image. Once the selection is made, the two non-selected images are replaced by a curved MPR image. The user draws the curve on the remaining, selected image. This is limiting, in that if the curve is particularly tortuous, the user is unable to place curve points in the centre of the curve path, so that the resulting curve and curved MPR image may not accurately reflect the user's desired view. The two non-selected images are also no longer available to use as references to determine curve orientation when the user becomes disoriented.
An alternative system [5] retains all three orthogonal views while the curved MPR image is displayed. The curve is shown in each of the views, and can be edited by manipulation of movable points. However, the user has only minimal control of a chosen curved sheet, and therefore can be easily confused about the orientation of the curve.
Thus there is a need for an improved curved MPR system.